Automatic protection of amplifier tubes



Aug. 7, 1934. J. n. WALLACE f 1,959,341

AUTOMATIC PROTECTION OF AMPLIFIER TUBES Filed Oct. 14. 1931 3 Sheets-Sheet l E ab gmc/nto@ Aug. 7, 1934. J. D. WALLACE AUTOMATIC PROTECTION OF AMPLIFIER TUBES FiledOcc. 14, 1951 3 Sheets-Sheet 2 ...Tw [L l avi/vento@ duca/nut Aug. 7,` 1934. J. D. WALLACE AUTOMATIC PROTECTION OF AMPLIFIER TUBES Filed Oct. 14. 1931 5 Sheets-Sheet 5 llll'lllllll 37 lllI tions of excessive rise of current in the output forth more fullylin the vspecification hereinafter Patented Aug. 7, 1934 PATENT OFFICE AUTOMATIC PROTECTION OF AMPLIFIER TUBES llames D. Wallace, Washington, D. C. Application October 14, 1931, Serial No. 568,838

8 Claims.

My invention relates broadly vto signal transmission systems and more particularly to a protective circuit for the power amplifier circuits of signal transmission systems; A

5 One ofthe objects of my invention is-to provide a protective circuit forv the power amplifier system of a signal transmitter to prevent injury to the power amplier when the plate circuit is out ofresonance with the oscillations from the associated high frequency generator. K

Another object of my invention Ais to provide thermally responsive means controlled by the tank circuit of 'a power amplifier for operating a relay system when the current in the tank circuit exceeds a predetermined amount for modifying the operating characteristics of the power amplifier for protecting the amplifier againstinjury.

`A still further object of my invention is to provide a thermal control device operated from the output of a power amplifier for changing the potential on certain of the electrodes of the amplifier under conditions of excess current vin the output circuit of the amplifier.

A further object of my invention is to provide a power amplifier system including a space charge grid tubev having means `automatically operative under control of the current in the output cir- ,cuitv of the space charge grid tube for modifying the potential on the space charge grid for protect.- 1 ing the tube against injury upon excessive rise of current in the output circuit of the space charge grid tube.

A still further object of my invention is to provide an arrangement of power amplifier tube including a shield grid having means automatically operative'under control of theV current in the output circuit of the amplifier for changing the potentiall of the shield grid for modifying the operating characteristics of the tube under condicircuit of the amplifier. y

Other andfurther objects of my invention reside in an automatic control circuit for the protection of electron tube transmitter circuits as set following by reference to the accompanying'drawings in which: y

` Figure 1 diagrammatically illustrates the circuit arrangement of my invention; Fig. 2 is a curve diagram explaining the principles of my invention; Fig. 3 shows my invention applied to an electron tube of the, space charge and shield grid-type; Fig. 4 illustrates my invention as applied to a three electrode electron tube circuit;

Fig. 5 shows a modified form of protective circuit i embodying my invention; Fig. 6 illustrates a modified form of protective circuit for an electron tube power amplifier system of the space charge and shield grid type; Fig. 7 shows a further modified'form of protective circuit embodying my invention; Fig. 8 illustrates an arrangement for controlling the grid bias potential of the power amplifier according to the current in the tank circuit of the amplifier; Fig. 9 shows a modied form of grid circuit control system for a l 'generator is indicated by reference character 1 and furnishes excitation voltage for the power amplifier 2. The plate circuit of this amplifier is composed of the inductance 3 and variable capacity 4. The output circuit of the amplifier is completed through the direct current meter 6, resistance '7 and the contacts 10 of the relay 8. The winding 11 of the relay 8 connects to the thermoelement 9 constituted by two dissimilar metals having their juncture connected in the tank circuit comprising the inductance 3 and variable capacity 4. The tank circuit includes meter 5 Vdisposed in series therewith as shown. l

The reading of the radio frequency current of ammeter 5 in the tank circuit and the reading of the direct current ammeter 6 in the output cir- 'cuit may be noted as condenserfi is varied, and

the resultsV plotted in the form of a curve as shown in Fig. 2. The abscissa points of the curves shown in Fig. 2 represent the setting of condenser land the ordinates denote the reading of the ammeters 5 and 6. The abscissa point A is the setting of the condenser 4 at which it and the inductance 3 are in parallel resonance at the excitation frequency. This parallel circuit is often referred to as a tank circuit. Curve B shows the tank circuit current as read by ammeter 5, and the curve C shows the D. C. plate current as read by ammeter 6. In Fig. 2 the maximum tank circuit radio frequency current is shown at the same tuning condenser setting that the D. C. plate current is at a minimum. These two conditions do not always occur at exactly the same condenser setting but the two settings are usually very close together if they do not coincide. An amplifier, especially if it be the last stage in the transmitter, is usually operated so that plate current taken by the amplifier when the plate circuit is in tune, is as much as the tube will accept without damage. The power put into an amplifier dissipated largely in three ways. Part is used heating the plate of the tube; another portion is used in the external plate circuit losses; and the rest is used in exciting the next amplifier stage or is dissipated by the antenna, depending on whether this particular stage is an intermediate or a final transmitter stage. In the operation of a transmitter amplifier, it is the usual thing to adjust the several electrode potentials and circuit characteristics until the amplifier takes as much power as possible without overheating of the plates when the plate circuit is in tune.

It will be observed from curve C in Fig. 2 that minimum plate current fiows when the external circuit is resonant at the impressed frequency. It has also been shown that only a portion of the plate input power is dissipated in heating the plate of the tube under this condition. It will also be seen from the curve C in Fig. 2 that the plate current increases greatly when the circuit is out of tune. When the circuit is out of tune, none of the plate power can be absorbed by the external circuits and consequently, all of the current is used in heating the plate of the tube. As a result, it is shown that power dissipated on the plate when the amplifier is out of tune may be many times what it is found to be when the circuit is properly tuned. Since it is customary to make the tube accept as much power as is possible and yet prevent damage to the ampliiier when the plate circuit is properly tuned, it is plainly seen that increasing the plate dissipation enormously for even a short time may ruin the amplifier, especially if it is a high power tube such as 250 watts or larger.

Heretofore it has been customary to place a protective resistance in the D. C. plate current circuit at some point to prevent the power dissipated on the plate of the tube from becoming too great while tuning up the transmitter. When the circuit is properly adjusted, the protective resistance is then short circuited to prevent power loss. In many types of transmitters, arrangements are made for operating the transmitter on many different frequencies, thereby necessitating a retuning of the transmitter circuits at frequent intervals. If the operator fails to insert the protective resistance before changing frequencies, damage to the amplifier tubes is likely to follow.

The circuit arrangement of my invention automatically inserts the protective resistance when the amplifier circuit is not in tune with the oscillator thereby making it unnecessary to manually insert and remove the protective resistance, and affording protection if the operator through negligence fails to insert the protective resistance shown by 7.

The electron tube 2 includes cathode 2a, control grid 2b, anode 2e and shield grid 2d. The source of high frequency energy 1 is coupled to the power amplifier 2 through the by-pass condenser 12. The bias potential for the control grid 2b is supplied from battery 13 through choke coil 15. The cathode 2o is heated from any suitable source such as battery 17. The output circuit is energized by battery 14 connected in series with the resistance 7 through meter 6 to the tank circuit 3, 4 and 5 which connects to the anode 2c. The potential source 14 has the point of potential 18 therein connected through rcsistance 19 to the shield grid 2d. A by-pass condenser 16 is connected between shield grid 2d and cathode 2a across the potential source which is applied to the shield grid 2d. Condenser 20 serves asaby-passpath across the potential source inthe output system of the power amplifier tube 2. One side of the oscillator and power amplifier system is connected to ground as shown at 21. A tap 22 is taken from the inductance 3 in the tank circuit and leads to the antenna or the succeeding stage of power amplification.

Reference character 9 designates a thermocouple constituted by two dissimilar metals at 9a and 9b forming a junction which will produce an electromotive force and a current when the junction is heated by the passage of radio frequency tank circuit current. From Fig. 2 it is seen that the radio frequency current passing through the thermo-couple heater 9 is inappreciable when the plate circuit is out of tune but when the plate circuit is in tune, the current heats one junction of the thermo-couple thereby producing enough power to energize the coil 11 of relay 8 and thereby close the contacts which short circuit the protective resistance '7 and allow the full plate voltage to be applied to the tube. In this manner, the amplifier is automatically protected at all times.

The circuit in Fig. 1 shows the shield grid pctential as remaining the same whether or not the protective resistor 7 is inserted. Under certain conditions, if the plate voltage is low, as would be caused when the protective resistor is inserted, the plate does not attract as many electrons as it normally would and consequently the shield grid attracts more. As a result the shield grid draws more power when the plate voltage is low and it is possible to exceed the shield grid power dissipation under this condition. For this reason, in connection with certain tubes, it is advisable to let the protective resistor decrease the shield voltage when it is inserted. The same line of rea.- soning follows in connection with the space charge grid, providing that type of tube be used.

The circuit in Fig. 3 shows a diagram of the connections used in applying the aforementioned improvements. A five element tube is shown at 2 having space charge grid 2e. The shield grid 2d renders the operation of the amplifier stable without the use of neutralization. The space charge grid has been designated at 2e connected through resistor 23 with the potential point 24 in the output circuit. The shield grid 2d connects as heretofore described through resistance 19 leading to the potential point 24 in the output circuit. The remaining parts of the circuit are the same as explained in connection with Fig. 1.

The automatic protective system of my invention as applied to a neutralized amplifier is shown in Fig. 4. If necessary, a space charge grid tube may be substituted for the three elements shown, provided proper resistance and connections are used to supply space charge grid potential. The three-electrode tube 2 shown in Fig. 4 has its control grid 2b connected through coupling condenser 12 to the source of radio frequency ener- .gy supplied from generator 1 which has its energy impressed across the tuned circuit constituted by inductance 25 and capacity 26. In order to neutralize the power amplifier tube with respect to inherent capacity coupling I provide a balancing condenser 27 connected between a. point in the exciting circuit and a point in the output circuit adjacent the anode 2c as indicated. Required adjustment can be made for balancing the operation of the power amplier system. The protective portion of the circuit constituted. by resistance 7 is connected 'into the circuit by the opening of contacts 10. Under normal operating conditions, however, the coil 1l remains energized from current generated in the thermocouple 9 and the contacts 8 remain closed as described in Figs. 1 and 3.

The systems hereinbefore described provide protection by decreasing the plate potential when the plate circuit is improperly adjusted, but protection may be automatically given by causing the shield grid potential to be lowered when the plate circuit is out of tune. The circuit in Fig. 5 shows a system of protection employing the shield grid method. The resistance 28 in this case is of proper value to cause the shield grid 2d to assume the proper potential for the proper operation of the amplifier. Resistance 28 1s `in series with resistance 7 when power is rst applied and remains so until the plate circuit is properly tuned. The resistance 7 is of such value that when in series with resistance 28 it causes the shield grid to have a low enough value of positive potential that insufficient plate current is drawn to cause any damage to the amplifier before it is properly tuned. When the plate circuit is in tune, the tank circuit current heats the thermo-couple 9 which produces suiiicient power to close the contacts 10 of relay 8 thereby short circuiting resistance 7, whereupon the shield grid 2d receives its proper potential for normal operation. A

If a five element tube is substituted for the tube shown in Fig. 5, it is advisable for the relay to control the space charge grid potential as it does the shield grid potential. If the space charge grid potential were not lowered at the same time the shield grid potential is decreased, the space charge grid would attract many electrons which might damage the tube because of excessive heating, because when the shield grid potential is low, neither the shield grid lnor the plate attract their normal number of electrons, and consequently the space charge grid would attract many more than normal.

The circuit in Fig. 6'shows vthe protective system as applied to a ve element tube. The protective series resistor is shown at 7 and the shield grid resistor at 19 as before; The space charge grid 2c is connected vthrough resistor 23 and resistor 'i to the source 14 as shown. The thermo-couple 9 and relay 8 operate as described heretofore.

A tube which has no shield grid may be substituted for the tubes sho-Wn in Figs. 5 and 6, but neutralization must be provided if the stage is not a frequency multiplier, however the neutralization circuits may be eliminated providing the amplifier is also a frequency multiplier. It is'understood that, if desirable, the circuit in Fig. 6 may have relay with two contactsand have two protective resistors in the circuit with thev space charge grid and the shield grid, each of which is short circuited by the relay contacts when the plate circuit is properly tuned.

The circuit shown in Fig; 7 gives the same kind of protection as was described in the preceding paragraph, and as was illustrated in Figs. 5 and 6, but illustrates a modified method of lowering the potential of the shield and space charge grids. The relay 8 in Fig. 7 is a back contact relay that is, the contacts 10 are closed when the relay coil 11 is not energized, and open when current is passed through the coil 11. The contacts 10 are disposed in series with resistance units 29 and 30 in a path connected in shunt to the source of potential 1li as shown. A potential point 31 in this shunt path connects through the resistors 19 and 23 with the shield grid and the space charge grid respectively. The other parts of the power amplifier circuit are arranged in the manner heretofore described. When the plate circuit is out of tune, the potential at the point 31 is reduced by the leakage path of the resistance 29 so that the potential applied to the screen grid 2d and the space charge grid 2e is `rendered subnormal. The tube resistance is thereby increased and the plate current is correspondingly limited. When the plate circuit is tuned, the thermo-couple 9 delivers power, energizes the relay coil 11, and opens the contacts 10, thereby breaking the path through the resistance 29. As a result, the voltage drop across resistance 30 is not so great, and the potential of the shield grid 2d and space charge grid 2e rises, thereby causing the plate power input to be normal. If a space charge grid tube is not used, the space charge grid and resistance 23 in Fig. 7 may be eliminated. In cases lwhere it is desirable to use a four element tube which has no shield grid, the shield grid and resistance 19 in Fig. 7 may be eliminated, if the ainplifier is used as a frequency multiplier; however, if the amplifier is not a frequency multiplier, proper neutralization circuits must be provided.

Protection may be given the amplifier by providing circuits which cause the control grid to assume a `high negative potential with respect to the filament when the plato circuit is out of tune. The circuit in Fig. 8 shows a method of protection by increasing the grid bias when the plate circuit is detuned. The circuit in Fig. 8 is used Where the transmitter is provided with a high voltage bias supply 32. The bias supply 32 is bridged by resistor elements 33 and 34 and a mid-tap 25 connected through contacts 10 and through the choke coil l5 to the control grid 2b. The actuating coilll of relay 8 is connected as heretofore described to the thermo-couple 9 ener.- gized by the tank circuit .9i-4. A resistor 35 is shown which will be placed in parallel with resistance 33 when the relay contacts are closed, thereby supplying the normal control grid potential with respect to the cathode. YTo protect the tube when the plate circuit is out of tune, relay 8 is released, causing the contacts 1i) to open. The grid then becomes more negatively biased. The remaining portions of the protective circuit are similar to those heretofore described.

The circuit in Fig. 9 may be used where a high bias voltage is not used with the transmitter but only a working bias voltage is provided. The source of bias potential has been indicated at 37 connected through the grid lead 35 and resistance 36 through choke coil 15 to the control grid 2b. The relay 8 has its contacts 10 connected across resistance 36 and arranged to shunt resistance 36 when Winding 11 of the relay is energized from thermo-couple 9, thereby placing a normal bias potential on the control grid 2b. When, however the current in the tank circuit falls below a predetermined safe limit, then relay 8 opens Athe contacts 10 and introduces the resistance 36 into the circuit for placing a high bias potential on the control grid 2b, so as to limit the plate current. The grid leak 35 has been shown, but it may be eliminated if desirable. When this system is used with transmitters which have no grid bias supply, the grid leak 35 may be used to furnish the bias, thereby eliminating the need of the grid bias battery 37 shown in the circuit in Fig. 9. The shield grid 2d and its associated circuits may be eliminated and neutralizing circuits used instead in either of the circuits shown in Figs. 8 and 9 if the amplifiers amplify the frequency impressed upon their grids; but if they are frequency multipliers, either the shield grid and its associated circuits nor neutralization circuits are necessaiy.

If a space charge grid tube is used in connec tion with the circuits illustrated in Figs. 8 and 9, the relay 8 must be provided with an additional set of contacts 38 as shown in Fig. 10, to remove part of the space charge potential when the control grid is at high negative potential or the increased electronic iiow to the space charge grid 2e may raise it to such a high temperature that it might be damaged from overheating.

The two sets of contacts of relay 8 shown at 10 and 38 are controlled by the energization of relay winding 1l from the thermo-couple 9. These two sets of contacts are simultaneously operated, the contacts 10 being disposed in the control grid circuit in shunt with resistance 36 and in series with resistance 35 and choke coil 15, as shown. The space charge grid 2e is connected in series with contacts 38 which serves to normally shunt the resistor 23 which is disposed in series with resistor 30 leading to a selected potential point in the output circuit of the power amplifier system. By simultaneously controlling the potential on the control grid and the space charge grid by means of the magnitude of the current in the output circuit, the tube may be protected.

Either of the methods heretofore described may be utilized to lower the space charge grid potential. The shield grid 2d and its associated eircuits may be eliminated from the amplifier circuit in Fig. 10, provided the ampliiier is also a frequency multiplier. However, if the amplifier operates at the frequency of its excitation, neutralization must be provided if the shield grid and its associated circuits are eliminated.

The thermocouple 9 and its heater is designed with a considerable thermal capacity which will maintain sufficient thermo-current iiowing to keep the relay coil l1 energized during normal keying intervals of a telegraphie transmission system. If the transmitter .is left idle for some time, it is necessary to hold the key down a short time before resuming transmission in order to heat the thermo-ccuple to generate the closing current for the relay coil 3a. When my protective system is used with a carrier wave telephone transmitter, no difculties of this nature are encountered.

The relays shown in all diagrams except Fig. 'l operate in such a manner that a spring holds the contacts apart when the magnetic circuit is not energized and the contacts close when current flows through the coil. The operation of the relay in Fig. '7 is discussed hereinbefore.

I have shown the protective system of my invention in connection with a single stage of aniplification in a transmitter. If desirable, any or all stages of amplification may be equipped with this protective device. No keying system has been shown in connection with any of the circuits but any type keying system may be used with any circuits illustrated provided it is compatible with the tube and other parts of the circuit.

I have found the circuit arrangements of my invention highly practical in construction and successful for the protection of power amplier systems. While I have described certain preferred embodiments of my invention, I desire that it be understood that modifications may be made and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. In a signal transmission system, a source of high frequency oscillations, a power amplifier, input and output circuits connected with said power amplifier, connections between said input circuit and said source of high frequency oscillations, a tank circuit including inductance and capacity elements tunable over a predetermined frequency range, a thermo-couple thermally connected in said tank circuit, a relay including a set of contacts and an actuating winding, ccnnections between said actuating winding and said thermo-couple, connections between said contacts and points in said output circuit i-or modifying the potential in said output circuit when said tank circuit is adjusted out of resonance with said source of high frequency oscillations.

2. In a signal transmission system, a source of high frequency oscillations, a power amplifier, input and output circuits connected with said power amplifier, connections between said input circuit and said source of high frequency oscillations, a tank circuit including inductance and capacity elements tunable over a predetermined frequency range, a thermo-couple thermally connected in said tank circuit, a relay including a set of contacts and an actuating winding, connections between said actuating winding and said thermo-couple, connections between said contacts and points in said input circuit for modifying the potential on the control grid of said power amplifier under conditions of too low a value of circulatory current in said tank circuit, as said tank circuit is adjusted out of resonance with said source of high frequency oscillations.

3. In a signal transmission system, a source of high frequency oscillations, a power amplifier, input and output circuits connected with said power amplifier, connections between said input circuit and said source of high frequency oscillations, a tank circuit including inductance and capacity elements tunable over a predetermined frequency range, a thermc-couple thermally connected in said tank circuit, a relay including a set of contacts and an actuating winding, connections between said actuating winding and said thermo-couple, a source of potential for said output circuit, a resistance disposed in series with said source of potential in said output circuit, connections across said opposite ends of said resistance to said contacts, whereby the eifective potential in said output circuit is modified upon too low a value of current in said tank circuit.

4. In a signal transmission system, a source of high frequency oscillations, a power amplifier constituted by an electron tube including a cathode, and an anode and at least one other electrede, an input circuit interconnecting said control grid and said cathode with said soiuce of high frequency oscillations, an output circuit interconnecting said anode and said cathode, a source of potential in said output circuit, a tank circuit connected with said output circuit and tunable over a variable frequency range, means It ZU for normally impressing a predetermined voltage from said source of potential upon said other electrode, and means controlled by too low a value of current in said tank circuit for decreasing the voltage on said other electrode until the space current between the said anode and said cathode is reduced to a predetermined safe value.

5. ln a signal transmission system, a source of high frequency oscillations, a power amplifier constituted by an electron tube including a cathode, a control grid, an anode, and at least one other electrode, an output circuit interconnecting said anode and said cathode, a source of potential in said output circuit, a tank circuit connected with said output circuit and tunable over a variable frequency range, means for normally impressing a predetermined voltage from said source of potential upon said other electrode, and means controlled by too low a Value of current in said tank circuit for decreasing the voltage on said other electrode until the space current between the said anode and cathode is reduced to a predetermined safe value.

6. In a signal transmission system, a source of high frequency oscillations, a power amplifier constituted by an electron tube including a cathode, an anode and at least one other electrode, an input circuit interconnecting said control grid and said cathode with said source of high frequency oscillations, an output circuit interconnecting said anode and said cathode, a source of potential in said output circuit, a tank circuit connected with said output circuit and tunable over a Variable frequency range, means for normally impressing a predetermined voltage from said source of potential upon said other electrode and means controlled by too low a value of current in said tank circuit for modifying the voltages impressed upon the respective electrodes until the space current between said anode and said cathode is reduced to a predetermined safe Value.

'7. In a signal transmission system, a source of high frequency oscillations, a power amplier constituted by an electron tube having a cathode, a control grid, and at least one other electrode, an input circuit interconnecting said control grid and said cathode with said source of high frequency oscillations, an output circuit for said tube, a tank circuit connected with said output circuit and tunable over a variable frequency range, a thermo-couple thermally ccnnected with said tank circuit, a relay including an actuating winding and a set of contacts, connections between said actuating winding and said thermo-couple, a source of biasing potential for said control grid, and connections between said contacts and means including said source of biasing potential for increasing the negative control grid potential with respect to said cathode when too low a value of current exists in said tank circuit.

8. A signal transmission system comprising a source of high frequency oscillations, a power amplier constituted by an electron tube including a cathode, a control grid, an anode at least one other electrode, an input circuit interconnecting said control grid and said cathode with said source of high frequency oscillations, an output circuit interconnecting said cathode and said anode, a tank circuit connected with said output circuit and including inductance and capacity elements tunable over a variable frequency range, and means for modifying the potentials applied to certain of the electrodes including said control grid whereby the space current between the anode and the cathode is protectively limited, said means comprising a thermo-couple thermally connected with a point in said tank circuit, a relay including an actuating winding and contacts controlled thereby, said actuating winding being connected with said thermocouple, a source of potential in said output circuit, and connections between said source of potential and certain electrodes of said tube including the control grid, said contacts being in circuit with said connections and the connection to said control grid constituting a biasing circuit within which is a resistance shunting said contacts.

JAMES D. WALLACE. 

